2006ApJ...637L..97B

Recent observations have discovered star formation activities in the extreme outer regions of disk galaxies. However, it remains unclear what physical mechanisms are responsible for triggering star formation in such low-density gaseous environments of galaxies. In order to understand the origin of these outer star-forming regions, we numerically investigate how the impact of dark matter subhalos orbiting a gas-rich disk galaxy embedded in a massive dark matter halo influences the dynamical evolution of the outer H I gas disk of the galaxy. We find that if the masses of the subhalos (M_sb) in a galaxy with an extended H I gas disk are as large as 10^–3M_h, where M_his the total mass of the galaxy's dark halo, local fine structures can be formed in the extended H I disk. We also find that the gas densities of some apparently filamentary structures can exceed a threshold gas density for star formation and thus be likely to be converted into new stars in the outer part of the H I disk in some models with larger M_sb. These results thus imply that the impact of dark matter subhalos (``dark impact'') can be important for better understanding the origin of recent star formation discovered in the extreme outer regions of disk galaxies. We also suggest that characteristic morphologies of local gaseous structures formed by the dark impact can indirectly prove the existence of dark matter subhalos in galaxies. We discuss the origin of giant H I holes observed in some gas-rich galaxies (e.g., NGC 6822) in the context of the dark impact.